Embodiments of the invention relate generally to ultrashort TE (UTE) MR imaging and, more particularly, to a system and method of calculating a phase image frequency map based on image data acquired during execution of a UTE MR scan.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Ultrashort echo time (UTE) MRI utilizes specialized pulse sequences with nominal TEs as low as a few microseconds to detect signals from the short T2 tissues frequently encountered in the musculoskeletal system. FIG. 1 illustrates an exemplary gradient recalled echo (GRE) sequence. As illustrated, the TE field 2 is defined as the time starting at the center of the RF pulse 4 and ending at the center of the data acquisition (DAQ) window 6 where k=0. In contrast, FIG. 2 illustrates an exemplary pulse sequence diagram of a UTE pulse sequence. As shown, a nominal TE field 8 is defined as the time from the end of the RF pulse 10 to the beginning of data acquisition 12 where k=0.
Using UTE sequences, it is possible to directly visualize short T2 tissues such as tendons (T2≈2 ms), ligaments (T2≈4-10 ms), menisci (T2≈4-10 ms), and cortical bone (T2≈0.5 ms). This is usually achieved by acquiring the Free Induction Decay (FID) of the MR signal as soon after the end of the RF excitation pulse as possible, and is frequently accomplished with a radial center-out k-space trajectory and data sampling of a few hundred microseconds. Magnitude images may be reconstructed from the (re-gridded) k-space data.
Susceptibility weighting is sometimes used as a source of contrast in studies of the brain and body employing gradient echo sequences with typical TEs of 10-40 ms to allow time for phase difference to evolve. In studies of short T2 tissues, much shorter TEs are used to detect signals. Accordingly, the time available to develop significant phase differences is more limited for UTE imaging. High phase contrast may be found, however, in phase images reconstructed from data acquired during a UTE sequence.
It would therefore be desirable to have a system and method capable of generating a frequency map based on a phase image reconstructed from data acquired during a UTE sequence.